1. Field of the Invention
The present invention relates to driving force controlling apparatus and method for a four-wheel drive vehicle in which a front-and-rear road wheel driving force distribution control system which variably controls a distribution ratio of a front-and-rear road wheel driving force according to an engagement control of a frictional clutch is mounted.
2. Description of the Related Art
Herein explained is, for example, an electronically controlled four-wheel drive vehicle on a front wheel drive basis. That is to say, when one of front road wheels is under an acceleration slip state during a vehicular run in a front wheel drive state on a low frictional coefficient (xcexc) road surface so that a wheel velocity difference between the front and rear road wheels occurs, such a front-and-rear wheel driving force distribution control as a clutch engagement force of the frictional clutch is carried out to suppress a velocity difference between the front and rear road wheel velocities and part of the engine driving force is distributed over the rear road wheel side so that a driving force distribution ratio toward the front road wheels is reduced is carried out. Then, in a case where the front-and-rear road wheel velocity difference is not yet suppressed even if the driving force distribution ratio toward the front road wheels is reduced, the clutch engagement force of the frictional clutch is furthermore increased. Finally, the clutch is engaged in such a manner that the driving force distribution ratio between the front wheels which are engine directly coupled drive wheels (hereinafter, referred to as main driven wheels) and rear wheels which are clutch engaged driven wheels (hereinafter, referred to as auxiliary driven wheels) is 50%:50%.
However, in order to meet a demand to secure a driving force transmitted on a road surface with the acceleration slip suppressed when the acceleration slip is suppressed when such a four-wheel acceleration slip state that develops acceleration slips on front and rear road wheels in spite of the fact that the vehicle is running on an extremely low frictional road surface in the four-wheel drive state, a traction control system (abbreviated as TCS) such that an engine output is reduced or a braking force is given to each road wheel has been applied to the four-wheel drive vehicle.
A Japanese Patent Application First Publication No. 2001-039287 published on Feb. 13, 2001 exemplifies a previously proposed driving force controlling apparatus for the four-wheel drive vehicle in which a vehicular body velocity is estimated on the basis of velocities of the auxiliary driven wheels by the traction control system and an occurrence of the acceleration slip is determined according to a deviation between estimated vehicular velocity and detected wheel velocities of the main driven wheels so as to execute a TCS (Traction Control System) control.
However, since, in the traction control system in the four-wheel drive vehicle, the vehicular body velocity is estimated on the basis of the detected road wheel velocities, a larger vehicular body velocity is estimated with respect to actual vehicular body velocity under the four-wheel acceleration slip state (hereinafter, referred to as the estimated vehicular body velocity). The wheel acceleration slip cannot appropriately be determined, a TCS operation timing is delayed, and only an insufficient acceleration slip suppression can be made even by the TCS operation.
In order to solve the above-described inconveniences, a limitation is placed in the driving force distribution ratio toward the auxiliary driven wheels. The limitation is, for example, the driving force distribution ratio toward the main driven wheels (90%) and the driving force distribution ratio toward the auxiliary driven wheels (10%)=90%:10%. In a case where the limitation is provided for the driving force distribution ratio toward the main driven wheels, slower wheel velocities are developed on the auxiliary driven wheels, a synchronization of the four-wheel velocities can be provided and an accuracy of the estimation of a vehicular body velocity can be improved. However, a natural vehicular acceleration performance that the four wheel drive state has and in which the driving force distribution rate toward the auxiliary driven wheels is high cannot be exhibited.
Furthermore, in a case where the limitation is placed on the driving force distribution ratio to the auxiliary driven wheels in the front-and-rear driving force distribution control system, the frictional clutch is under a slip state and a temperature in the frictional clutch is accordingly raised, this repetitive operation of the frictional clutch gives a disadvantage against a durability in the frictional clutch.
It is, hence, an object of the present invention to provide driving force controlling apparatus and method for the four-wheel drive vehicle which can achieve both compatibilities between assurances of the vehicular acceleration performance that the four-wheel drive vehicle naturally has and an estimation accuracy of the vehicular body velocity at a high level while improving the durability in the frictional clutch.
According to one aspect of the present invention, there is provided a driving force controlling apparatus for a four-wheel drive vehicle, comprising: a front-and-rear wheel driving force distribution control system including a clutch disposed in a midway of an engine driving system of the vehicle through front and rear road wheels of the vehicle, the front-and-rear wheel driving force distribution control system executing a control to modify a driving force distribution ratio between the front and rear road wheels toward an equal-distribution ratio as a clutch engagement force of the clutch becomes stronger; a road wheel velocity detecting section that detects a road wheel velocity of each of the front and rear road wheels of the vehicle; a clutch input torque detecting section that detects a clutch input torque inputted from the engine driving system to the clutch; a clutch transmission torque detecting section that detects a clutch transmission torque which the clutch is enabled to transmit to an output portion of the clutch; and a clutch force reduction controlling section that outputs a command to the front-and-rear wheel driving force distribution control system to reduce the clutch engagement force of the clutch when a subtraction value of a detected value of the clutch transmission torque from that of the clutch input torque is smaller than a predetermined value and detected wheel velocities of both of left and right road wheels of the vehicle are substantially equal to each other.
According to another aspect of the present invention, there is provided driving force controlling method for a four-wheel drive vehicle, the four wheel drive vehicle comprising a front-and-rear wheel driving force distribution control system including a clutch disposed in a midway of an engine driving system of the vehicle through front and rear road wheels of the vehicle, the front-and-rear wheel driving force distribution control system executing a control to modify a driving force distribution ratio between the front and rear road wheels toward an equal-distribution ratio as a clutch engagement force of the clutch becomes stronger and the driving force controlling method comprising: detecting a road wheel velocity of each of the front and rear road wheels of the vehicle; detecting a clutch input torque inputted from the engine driving system to the clutch; detecting a clutch transmission torque which the clutch is enabled to transmit to an output portion of the clutch; and outputting a command to the front-and-rear wheel driving force distribution control system to reduce the clutch engagement force of the clutch when a subtraction value of a detected value of the clutch transmission torque from that of the clutch input torque is smaller than a predetermined value and detected wheel velocities of both of left and right road wheels of the vehicle are substantially equal to each other.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.